1. Field of the Invention
The present invention relates to a method and apparatus for forming a thin film by a plasma enhanced chemical vapor deposition method (plasma CVD method), and more particularly to a method and apparatus for forming a thin film, in which an amorphous carbon thin film is formed while a deposition of adhesion on the inner wall of a reaction chamber is prevented.
2. Description of Related Art
In manufacturing of a semiconductor device, an amorphous carbon thin film is used as a low dielectric constant insulation material and so on and is formed by the plasma chemical vapor deposition method for example. FIG. 1 is a cross sectional view showing the structure of a conventional parallel plates type plasma enhanced chemical vapor deposition apparatus which is used to form the amorphous carbon film.
Referring to FIG. 1, in the chemical vapor deposition apparatus, a reaction chamber is composed of a support base 111, a cylindrical side wall 112 which is arranged on the support base 111, and an upper lid 113 provided to oppose to the support base 111 and to close the other end of the cylindrical side wall 112. An exhaust pipe 118 is attached to the side wall 112 to be connected to a vacuum pump 117. A gas introducing pipe 121 for introducing a material gas into the reaction chamber penetrates the side wall 112. The gas introducing pipe 121 has an opening in the reaction chamber at one end and the other end is connected via a control valve 120 to a gas cylinder 119 as a gas supply source. A lower plate electrode 114 and an upper plate electrode 115 are arranged in parallel to each other and to oppose to each other in the reaction chamber. A substrate 122 on which the amorphous carbon film will be formed is mounted on the lower electrode 114. The upper electrode 115 is grounded and a predetermined voltage is applied to the lower electrode 114 by a high voltage power supply 116.
When an amorphous carbon thin film is formed using the plasma enhanced chemical vapor deposition apparatus, the pressure in the reaction chamber is reduced to a predetermined value by the vacuum pump 117. At the same time, a material gas is supplied from the gas cylinder 119 into the reaction chamber through the gas introducing pipe 121. Then, high frequency electric power is applied between the upper electrode 115 and the lower electrode 114 from the high voltage power supply 116 so that high frequency plasma discharge can be generated. As the material gas, there is used a gas which contains, for example, a hydrocarbon gas or a carbon fluoride gas as a main component. At this time, the side wall 112 is kept at a temperature equal to about room temperature. As a result, an adhesion is deposited on the inner wall of the reaction chamber.
This adhesion acts as a generation source of an impurity gas when a semiconductor thin film is manufactured. Also, if the adhesion is peeled down from the inner wall of the reaction chamber onto the semiconductor substrate during forming of the thin film, defects are caused which form a pattern on the substrate, resulting in a decrease in the manufacturing yield. Further, the film quality of the thin film formed changes, because the state of the plasma to be generated differs between the case that there is not adhesion on the inner wall of the reaction chamber and the case that there is the adhesion on the inner wall, so that active particles differ.
Actually, the deposition of the adhesion of this type and the generation of particles derived from the adhesion have been a general problem in the chemical vapor deposition method. Conventionally, the problem has been avoided by mechanically wiping the inner wall of the reaction chamber using organic solvent and so on to remove the adhesion. Alternatively, the inner wall of the reaction chamber is made to be detachable and the inner wall is replaced when the semiconductor substrates of a predetermined number is processed. Further, there has been used a method in which etching plasma is generated in the reaction chamber so that the adhesion on the inner wall is removed.
For example, there is disclosed in the Japanese Laid Open Patent Disclosure (Heisei 3-82020) the technique in which a ring member is provided in a reaction chamber of a thermochemical vapor deposition apparatus and an inner wall of the reaction chamber is heated such that the ring member is maintained at a temperature lower than that of the inner wall, so that a fine particle reaction product is made to be adhered to the ring member by thermal migration, resulting in suppression of deposition of the adhesion to the other part of the inner wall.
In the Japanese Laid Open Patent Disclosure (Heisei 3-183128) there is disclosed the technique in which an electrode for removing adhesion and a movable separating member are provided in a reaction chamber of the plasma chemical vapor deposition apparatus and adhesion deposited on the separating member is removed by plasma cleaning using the electrode for removing the adhesion.
In Japanese Laid Open Patent Disclosure (Heisei 3-211279) there is disclosed the technique in which in an atmospheric chemical vapor deposition apparatus in which silane gas and oxygen gas are introduced to form a SiO.sub.2 thin film, an exhaust duct and gas dispersion head are heated to 200.degree. C. to 300.degree. C. so that deposition of adhesion (powder of SiO.sub.2) to the exhaust duct and gas dispersion head is reduced.
In Japanese Laid Open Patent Disclosure (Heisei 4-152515) there is disclosed the technique in which in a decompression chemical vapor deposition apparatus, which is constituted such that a reaction pipe is he ate d to a temperature as high as a thin film formation temperature, unevenness from 10 .mu.m to 500 .mu.m is provided in the inner wall of the reaction pipe so that a film deposited on the inner wall of the reaction pipe can be prevented from peeling.
In Japanese Laid Open Patent Disclosure (Heisei 4-186615) there is disclosed the technique in which a third electrode is provided in a reaction chamber in a plasma chemical vapor deposition apparatus a nd plasma etch ing is performed using the third electrode so that adhesion is removed from the inner wall of the reaction chamber.
In Japanese Laid Open Patent Disclosure (Heisei 4-262530) there is disclosed the technique in which in a thermochemical vapor deposition apparatus in which the first reaction gas (e.g., tetraethoxysilane gas) and ozonic (O.sub.3) gas are introduced to form a thin film of SiO.sub.2, deposition of adhesion is reduced by introducing a second reaction gas (e.g., ethylene gas) reacting with oxygen radicals and by heating the wall of the reaction chamber.
In the Japanese Laid Open Patent Disclosure (Heisei 5-211125) there is disclosed the technique in which in a thermochemical vapor deposition apparatus, the inner wall of a reaction chamber is heated to 50 to 200.degree. C. such that the adhesion deposited on the inner wall of the reaction chamber is sublimated and removed in vacuum.
Further, in Japanese Laid Open Patent Disclosure (Heisei 5-217910) there is disclosed the technique in which in a thermochemical vapor deposition apparatus in which a compound semiconductor thin film of GaAs or the like is formed using a reaction pipe having the double pipe structure to circulate cooling water, the reaction pipe is divided in three portions along the direction in which a reaction gas flows such that there can be removed the adhesion deposited on the inner wall of the middle one of the three divided portions of the reaction pipe by flowing cooling water through both of end portions of the reaction pile and by heading the middle portion.
As mentioned above, in the plasma enhanced chemical vapor deposition apparatus, it is required to regularly remove the adhesion deposited on the inner wall of the reaction chamber.
For this purpose, the apparatus must be maintained for every predetermined time period. Further, the method is performed in which the conditions of temperature and reaction gas are changed such that the deposition of unnecessary adhesion can be reduced. In the above methods, however, the conditions must be set in accordance with a kind of film to be formed and so on. The condition when an amorphous carbon thin film is formed by the plasma enhanced chemical vapor deposition method does not yet become clear.